Prior art track-and-hold circuits with very wide bandwidth suffer sample amplitude accuracy degradation due to the 2nd order effects of non-linearity and hold-mode signal feed-through. This results from the design tradeoffs between these parameters. Designs that offer very wide bandwidth with high linearity tend to exhibit high levels of hold-mode signal feed-through resulting from poor switch off-state isolation.
One prior art track-and-hold circuit is disclosed in U.S. Pat. No. 6,954,168, which is incorporated herein by reference. The track-and-hold circuit disclosed in the '168 patent set new performance benchmarks for a track-and-hold circuit with 4 GHz of input bandwidth.
With the track-and-hold circuit of the '168 patent, the tradeoffs between bandwidth, linearity, hold-mode droop and hold-mode feed-through become more significant as the track-and-hold circuit bandwidth increases. To obtain high linearity, a typical switched emitter-follower (SEF) track-and-hold circuit requires large transistors in the emitter-follower of its switching circuit, but these large transistors increase capacitive loading and reduce the bandwidth of the driving stage. These larger transistors also result in a substantial off-state coupling capacitance to the hold capacitor contributing to substantial hold-mode feed-through. Hence, increased linearity in a switched emitter-follower track-and-hold circuit is usually obtained at the expense of bandwidth, droop, and hold-mode feed-through.
A larger bandwidth is also desired for existing track-and-hold circuits. A track-and-hold circuit with increased bandwidth also has general application to high-speed analog-to-digital conversion. The ability to convert signals at X-band (˜10 GHz) is a common goal for several military systems. Hence, there is considerable interest in the military community in developing a track-and-hold circuit with 10 GHz, and larger, bandwidth.